Displacement of Organic Soil

[54Civil]

We are constructing a 8m high fill for a bridge approach. Under a portion of the fill there is a mined out gravel pit approx 2-3 m deep. Some organics have accumulated in the bottom. We are only filling a portion of the old pit. Does any one have a spec for placing granular fill to maximize the displacement of the organics without excavating them?

 

[Ron]

Organic material will degrade and settlement will occur as a result. Further, if the material is an organic silt, you'll get long term settlement even before the "rotting" takes place. Not a good idea to leave it in place. Trying to displace with other material will likely only contaminate the other material. Take it out.

 

[BigH]

I think that he is asking about "rolling surcharge" - a technique described in Tschebotariof (1951) book. It comes from a 1947 Michigan Dept of Highways paper or procedure. - I used it before - basically, you create a mound of sand at the edge of the organic - build it up and use a dozer to cause the outer slope to fail - but as it slides down the sand, it will force out, in a wave, the material that was very soft. We displaced about 13 m of tailing slimes in a Guyanese tailings facility. It think it is also described in a paper by MAJ Matich for a Sudbury Ontario mining conference back in the 1980s. I'll try to find it.

 

[fattdad]

I was working on a project where we reclaimed land from a sewage lagoon. The contractor just end-dumped fill dirt (silty sand) from the edge and backed his way onto the fill as the end-dumping progressed. Sure, there was some trapped organic matter below the end-dumped fill. Sure, there was no control on compaction, but the dike the we created was strong enough for dewatering the reclaimed land area.

When the relclaimed land area was dewatered, the contractor did his best to remove the sludge. As the fill was placed, there was a displacement effect that created a "mud wave" that he then collected as needed.

I think there are details of the OP that are unknown. First how thick is the accumulation of organic matter? I mean if it's 6 or 8 inches and then you have compact sand and gravel, the risk seems low. If it's 6 or 8 inches and then below there is a natural layer of normally consolidated fat clay, then the risk is high (well, the problem being the natural soils rather than some thin accumulation of organic matter. Alternatly, if it's 5 ft thick that'd be a different matter (i.e., you may not have the advantage of some mud-wave displacement effect.

Just some ramblings. . .

f-d

¡papá gordo ain’t no madre flaca!